When creating mobile app, any resource files with deploy action in data tab must be reachable from client currently. The reachability would be required even if a resource files was needed only by server actions. If the resource file had a non-standard extension, this extension should have to be appended to MIME settings of the web server for the reachability from client.  If there is an 'only for server action' flag, the reachability will become to needless.

I talked about this in the following thread:
https://www.outsystems.com/forums/discussion/34717/mobile-apps-resource-file-with-a-non-standard-extension/